1. Field
The invention relates to a method and a measuring device for measuring an object by radio-frequency electromagnetic radiation.
2. Description of the Related Art
Measuring by radio and microwave frequencies may employ as a sensor a resonator that is designed such that characteristics of the object to be measured affect the resonance frequency of the resonator. Because, in general, only one characteristic is measured, the objective is, that at least approximately, the object to be measured has only one factor that affects the resonance frequency. When the frequency of electromagnetic radiation to be applied to the resonator is scanned over a frequency band used for measuring, a resonance frequency may be searched for in the response signal received from the resonator. In order to be able to determine the desired characteristic of an object to be measured by means of the resonance frequency, prior to actual measuring it is necessary to carry out calibration measurements, in which objects to be measured have predetermined characteristics. In this manner it is possible to set correlation between various resonance frequency values and values of characteristics to be measured.
In prior art the measuring is performed such that a band to be measured is scanned over by a VCO (Voltage Controlled Oscillator) or a YIG (Yttrium-Iron Garnet) oscillator. The resonator output is detected, amplified and converted from analog to digital. From a digital signal it is possible to determine medium frequency, half-power width, Q-value and/or signal level of the resonator.
Measuring involves several problems, however. Measuring is most accurate immediately after calibration, but measuring error increases the higher the more time passes or the more measurements there are performed after calibration. The error may be reduced by performing more frequent calibrations, but this, in turn, decreases, slows down or even disturbs the actual measurement use. The band to be measured, which is scanned over, by the VCO and the YIG oscillator, is excessively narrow for many measurements. In addition, scanning over the band to be measured takes place too slowly in view of many applications.